Modern day jet airplanes direct regulated airflow from the jet engine to the occupied cabins and other areas of the aircraft. This airflow, commonly referred to as bleed air, may be withdrawn from the high pressure compressor (HPC) section of a jet engine. U.S. Pat. Nos. 5,137,230 and 5,125,597 describe conventional structures and methods utilized to direct bleed air into environmental control systems (ECS) of the aircraft that further process the bleed air prior to cabin introduction. ECS incorporate various pieces of equipment such as air cycle machines (ACMs), regulating valves, heat exchangers, and other apparatus to condition engine bleed air prior to cabin introduction.
Bleed air is commonly extracted from multiple locations along the HPC section using regulated flow to control the extent to which bleed air is withdrawn. Among the regulating structures are check valves that operate to allow or discontinue airflow and downstream regulator valves that reduce the pressure of the withdrawn bleed air before it reaches the ECS. This reduced pressure bleed air may be directed to a turbine, where work is extracted, with the bleed air outlet pressure and temperature from the turbine being significantly reduced. This reduced pressure bleed air remains relatively hot and is thereafter cooled by fan air in a heat exchanger associated with the jet engine conventionally referred to as a precooler. Cooled bleed air output from the precooler is delivered to the ECS where it may be further cooled and pressures further regulated prior to introduction to the occupied cabins or other areas of the aircraft. In addition to supplying bleed air to the ECS, the jet engine provides a heat sink that provides precooled air to the aircraft and receives high temperature air from the aircraft in return as part of a cycle.
Regardless of the structures or methods utilized, one constant has remained with respect to the bleed air supplied to the ECS: it could be no lower in temperature than the lowest temperature air flowing through the jet engine. Moreover, the bleed air has always been regulated from the HPC using flow control valves that restrict airflow and are operative to step down the bleed air pressure prior to reaching the precooler. Consequently, there is a need in the art for structures and methods of delivering bleed air to an ECS at temperatures lower than the lowest temperature air otherwise flowing through the jet engine.